The present invention relates to the surface profiling of materials, for example as in the laser processing or ablation of materials, or as needed in the calibration and performance checking of the laser apparatus used in operations on the corneal tissue of the eye for the correction of refractive errors.
The invention will be described by reference to two operations for the correction of refractive errors, photorefractive keratectomy (PRK) and laser in-situ keratomeleusis (LASIK), but the invention may be used to measure the surface profile of a wide range of materials or to calibrate lasers for a variety of medical and industrial applications.
To ensure that the correct profile is etched onto a patient""s cornea during PRK or LASIK, the surgical laser must first be calibrated. This process imparts an accurate picture of how the laser will ablate the cornea. The corneal surface may be ablated to effect a myopic, hyperopic or astigmatic correction. Myopic corrections should produce a new, flatter curvature, while hyperopic corrections should remove more material around the edge of the area to be ablated.
One of the current methods used to perform the calibration procedure involves etching the surface of a plastic polymer such as polymethyl-methacrylate (PMMA). The etched surface is examined by an instrument known as a lensometer. This instrument determines the power of the resultant xe2x80x98lensxe2x80x99 in diopters. The reading can then be compared to the desired refractive correction. Discrepancies between the desired and achieved readings indicate that the laser needs to be adjusted, by a factor proportional to the difference between the lensometer reading and the desired surgical correction (see U.S. Pat. No. 5,261,822).
Another method of calibration is described in U.S. Pat. No. 5,261,822. This patent illustrates the use of a calibration block that can be examined by visual inspection. It teaches the use of a plurality of thin coatings of PMMA of progressively increasing thickness, layered over a solid substrate of the same material. Each layer may be doped with a differently coloured or fluorescent material. When the cavity of material ablated by the laser is viewed from above, a pattern of circles is visible. A correctly calibrated laser should produce patterns of concentric circles, whereas patterns of eccentric circles indicate that the laser is not correctly calibrated. However, the result is usually judged subjectively and this technique provides only a crude prediction of the shape created during a refractive correction.
The above laser calibration methods suffer from a number of disadvantages. PMMA does not necessarily mimic the ablation characteristics of corneal tissue, and different brands of PMMA ablate at different rates (P.P. van Saarloos and I. J. Constable, J. Appl. Phys. 68(1) (1990) 377). Further, different brands of lasers ablate at different fluences, where the ratio of ablation rates of tissue and plastic are different. Nor does the lensometer provide an accurate reading of the ablation surface. The shape desired to be etched on the cornea does not necessarily produce an accurate lens shape when ablated into plastic. The ablated surface is usually aspheric, and may be inaccurately read. This means that a lensometer reading does not give an absolute measure of laser performance, and in some cases the measurement is meaningless. This method can therefore only give an approximate reading of surface curvature. Lensometer readings are also time consuming.
Other known methods to measure ablated surface profiles include the use of interferometry, or include scanning the ablated surface with a scanning electron microscope, a confocal microscope or surface contact needles. Devices according to these known methods are, however, costly and of prohibitive size, and impractical to cover the range of shapes produced by refractive lasers. There exists, therefore, a demand for an accurate, low cost device for performance analysis and calibration of refractive lasers to ensure appropriate shapes are etched onto the surface to be ablated.
It is an object of the present invention to provide a new and improved method and apparatus for surface profiling of materials and calibration of ablation lasers that can more accurately and reliably examine the surface of an ablation.
According to the present invention, therefore, there is provided a method for measuring the surface profile of a sample, said method including:
directing light from a light source through a beam splitter to form two split beams;
directing said split beams onto a sample surface and a reference surface respectively;
reflecting the split beams back through the beam splitter; and
directing said split beams towards an imaging system.
Preferably the method is for use in calibrating a laser ablation apparatus for ablation of a material by measuring the result of an ablation of the sample.
The method may include reflecting said light from a mirror and/or focussing said light to minimise space requirements.
Preferably the light source includes a light emitting diode.
Alternatively the light source is a source of white or near infra-red light.
Preferably the sample surface is a plastic polymer that ablates at a substantially constant fraction of the ablation rate of said material over the range of fluences used in ablating said material, and preferably the fraction equals 1.0.
The material may be biological material.
The biological tissue may be corneal tissue, and the method include ablating said material in a surgical procedure, in which case the fluences are preferably in the range 50-800 mJ/cm2, and more preferably in the range 120-250 mJ/cm2.
Preferably the reference surface is a flat mirror or a flat surface.
The reference surface may be mounted on a pendulum including a plurality of substantially parallel sheets of flexible material.
The method may include moving the reference surface by means of a speaker or voice coil.
Preferably the imaging system includes a CCD video camera.
The method may include measuring said surface profile, comparing said measurement with a predicted profile, and determining an indicator of the safety or predictability of ablation performed on said sample for use in a surgical procedure.
Preferably the reference surface positioning means includes a voice coil driver and a position sensor.
The method may include transferring the calibration profile information ascertained by said method into a laser system control computing device, to allow the self correction of the calibration and shape controls of the laser system.
The method may also include communicating with a topography measuring device for measuring the topography of the front surface of a human or animal eye in order to combine the results of a calibration measurement in plastic and the results of a topography measurement, and predicting from said calibration and topography results the post laser treatment shape of the eye.
The present invention also provides a surface profiling apparatus for measuring the surface profile of a sample, the apparatus including:
a light source for generating a source beam;
beam splitting means positioned in the path of the source beam for splitting said source beam into split beams;
a reference surface;
a sample surface allowing said split beams to traverse separate paths and return to said beam splitting means;
reference surface positioning means for positioning the reference surface; and
viewing means for imaging combined beams.
The apparatus may include focussing optical elements to concentrate the intensity of said light, and a mirror, said optical elements and said mirror located between said light source and said beam splitting means.
Preferably the light is white light or near infra-red light.
The light source may include a halogen bulb, or a light emitting diode (LED).
Preferably said LED has a maximum intensity in the red to infra-red portion of the spectrum.
The reference surface may be a flat mirror or a flat surface.
The imaging system preferably includes a CCD video camera.
Preferably the reference surface positioning means includes a voice coil driver and a position sensor.
Preferably the position sensor includes a known sample.
Preferably the position sensor includes a mirror or optical element that allows both the known sample and the plastic sample being measured to be viewed by means of the imaging system simultaneously or alternately.
In one form of the invention, the position sensor is a capacitance or inductance position sensor.
Preferably the voice coil driver is similar to that used in a loud-speaker.
The position sensor may be an opto-electric sensor including a photodiode with an amplification system and an additional LED, wherein the sensor uses the intensity of the additional LED, and said additional LED is positioned to reflect light at an angle from the reference surface, or any surface moving with the reference surface, to the photodiode.
Preferably the position sensor is one of a plurality of position sensors.
Preferably the plurality of position sensors includes a plurality of types of position sensor.
In one embodiment, the reference surface positioning means includes a loud-speaker.
Preferably the loud-speaker is used as or constitutes a displacement driver for the reference surface.
Preferably the reference surface is mounted on a pendulum including a plurality of substantially parallel sheets of flexible material.
The invention also provides an apparatus for calibrating a laser for the ablation of a material including the surface profiling apparatus described above.
The sample surface may comprise a plastic polymer that ablates at a substantially constant fraction of the ablation rate of said material over the range of fluences used in ablating said material, and preferably the fraction equals 1.0.
The material may be biological material, including for example corneal tissue, and the apparatus may be for ablating the material in a surgical procedure (such as PRK or LASIK). In these cases the fluences are preferably in the range 50-800 mJ/cm2 and more preferably in the range 120-250 mJ/cm2.
In one particular embodiment, the apparatus includes a laser means, wherein the apparatus is for calibrating and/or checking the laser means, and includes communication means for communicating with, a computer controlled laser means, whereby the laser means can use calibration profile information obtained by the calibration apparatus to self correct the calibration and shape controls of said laser means. In this embodiment, the laser means may be for use in PRK or LASIK operations of the cornea of the eye to correct refractive errors.
The apparatus may include a corneal topography measuring means for measuring the topography of the front surface of a human or animal eye and communication means for communicating with said topography measuring means, for predicting post laser treatment eye topography from calibration measurements in plastic and topography measurements of the eye, and may further include display means for displaying the post laser treatment corneal topography predicted by means of the apparatus.